Genetic variations occur in susceptibility to infection in Biomphalaria glabrata and infectivity in Schistosoma mansoni. Studies indicate at least 12 genetically different susceptibility types in B. glabrata. Three S. mansoni isolations from chronic Puerto Rican cases believed to have been reinforced, one isolation from St. Lucia, and one from Egypt have each yielded by selection two genetically different substrains of S. mansoni. A gene regulating mantile pigment in B. glabrata has three alleles; mantle spotting, coalesced mantle pigment, and absence of mantle pigment. Absence of pigment is recessive, and snails heterozygous for spotting and coalescence show a combined expression of both alleles, so this gene involves four phenotypes. Since the gene for albino, blackeye, or wildtype pigmentation is located on a different chromosome, we can now distinguish nine different pigment phenotypes serving as genetic markers in B. glabrata. In some genetic types of B. glabrata, snails are susceptible as juveniles, resistant during the period of maximum egg production, and revert to susceptibility in old age. This is pertinent to snail and parasite survival in an endemic area, to studies on the mechanisms of resistance, and to experimental research and field surveys.